How do I make sure he's not cheating on me?

My boyfriend and I are in a long distance relationship. Some months ago, we met in a social dating event where we got along so much and date a lot. We communicate through calls in skype, even visit each other when we have enough time. But just recently, our daily video calls is not happening already. He said he's too busy with his work. But the thought that he might be seeing another woman is creeping in my mind. How do I make sure he's not cheating on me?